Louise Françoise Le Blanc de La Vallière is a second-year student at the Tristain Academy of Magic and the third daughter of the Vallière family, a prominent noble house in the kingdom of Tristain. She is known throughout the academy by the derogatory nickname Louise the Zero because every spell she attempts results in a spectacular explosion rather than the intended effect, giving her a reputation for complete magical failure. Despite this, she is fiercely proud of her noble status and harbors a strong sense of duty, particularly toward Princess Henrietta, whom she considers it her responsibility to protect. Her outward demeanor is arrogant, hot-tempered, and often bossy, but this masks deep insecurity and hurt feelings stemming from her inability to perform magic like her peers. She is also shown to be genuinely kind and caring beneath her prickly exterior.

Her motivations are driven by a desire to prove herself worthy of her family name and to overcome her shortcomings as a mage. When she summons a human from Earth, Saito Hiraga, as her familiar during a ceremony, her initial treatment of him is harsh and dismissive. However, as the story progresses in The Familiar of Zero: Rondo of Princesses, their relationship evolves from master and servant into a deep romantic bond. Louise becomes fiercely jealous whenever Saito pays attention to other women, often punishing him with her riding crop or a blast from her magic. Yet she gradually falls in love with him, and her growth as a character is closely tied to her evolving feelings for Saito and her increasing mastery of her true magical abilities.

It is revealed during the Albion invasion that Louise is not a failed mage at all but one of the rare Void mages, descended from the founder Brimir. Her inability to cast elemental magic is because she possesses the lost Void affinity. Her primary Void spell is Explosion, a powerful destructive force capable of leveling armies, though it requires time to cast. She also learns the World Door spell, which allows her to transport herself and others across dimensions. Her discovery of her true nature marks a turning point in her development, transforming her from a source of ridicule into a key figure in the conflicts of Halkeginia. Over the course of the series, she sheds her earlier insecurities, takes on greater responsibilities, and eventually marries Saito, using her World Door ability to travel with him to his world. Her role in the story is central as both the emotional core and a formidable mage whose growth embodies the themes of self-acceptance and the value of perseverance.